Many studies have been widely carried out to develop light emitting device which radiates a mixed-color light and luminaire using the light emitting device so far. The light emitting device includes an LED chip and a phosphor. (e.g., Reference 1) The phosphor acts as a wavelength conversion member which is excited by a light emitted from LED chip and emits a light of color different from the luminescent color of the LED. The light emitting device is capable of radiating a light of mixed-color which is different from the luminescent color of LED, when including the phosphor and the LED chip. For example, it is known that the blue-color or UV LED chip can be combined with the phosphor to give a light emitting device which radiates a white-color light (white-color luminescent spectrum).
For example, the light emitting device radiating white-color light is combined with each other for being applied to a luminaire. However, in this luminaire, a plurality of the light emitting device is required to be disposed on the circuit substrate, and accommodated within the main body. As a result, this luminaire may suffer from an excessive thermal resistance between the LED chip and the main body. In view of this problem, this light emitting device luminaire needs to be controlled to give a limited input electric power to the LED chip such that a junction temperature of the LED chip is kept below the maximum of junction temperature. As a result, this light emitting device may give a limited light output.
In the light emitting device using LED chip, the LED chip is required to be mounted on a highly thermal conductive mounting member, in order to smoothly dissipate outward heat and achieve an improved light output. The mounting member can be formed of a sub-mount member or a ceramic substrate which is secured to the heat conducting plate to relieve a stress acting on the LED chip due to a difference of coefficient of linear expansion between the LED chip and the heat conducting plate.
For example, Japanese unexamined patent publication 2007-149976 discloses a method of eutectically bonding the LED chip and a patterned conductor of the mounting member by way of an eutectic alloy layer (e.g., AuSn layer) which is disposed on rear surface of the LED chip, in the process of mounting the LED chip to the mounting member.
For example, Japanese unexamined patent publication 2007-149976 discloses that the LED chip is mounted to the mounting substrate by means of a reflow apparatus, after both an eutectic alloy layer provided on the LED chip and the mounting substrate are wetted with the flux. For example, it is known that the LED chip is moved closer to a patterned conductor provided on the mounting substrate and then connected thereto through the eutectic bond by means of a die bonding apparatus, while the mounting substrate is heated at its bottom surface by using a heater.
In this method of mounting the LED chip in Japanese unexamined patent publication 2007-149976, the eutectic bond can be free from void generation. But, this process needs the use of flux, possibly leaving residues on the mounting member.
In the method of mounting LED chip by means of the die-bonding apparatus, the eutectic alloy layer provided on a rear surface of the LED chip is moved closer to a patterned conductor provided on the mounting substrate. In this method, external air may be entrapped between the eutectic alloy layer provided on a rear surface of the LED chip and the patterned conductor provided on the mounting substrate, degenerating thermal conductivity between the patterned conductor provided on the mounting substrate and the eutectic alloy layer. Besides, the eutectic bond may undergo ununiform temperature due to a partial sudden boiling when the melted eutectic alloy layer comes to contact with the patterned conductor, suffering from void-generation inside the eutectic bond, and eventually increasing the thermal resistance between the LED chip and the mounting member. In addition, it is difficult to suitably control temperature and time of heating the melted eutectic alloy layer in accordance with thermal resistance of the LED chip. (For example, the eutectic alloy layer made of AuSn needs to be heated for five seconds or less, when the heating temperature is set at 325° C.)